This invention relates to a method and apparatus for separating air.
The most important method commercially for separating air is by rectification. In typical rectification processes, there are performed the steps of compressing a stream of air, purifying the resulting stream of compressed air by removing water vapour and carbon dioxide from it, and pre-cooling the stream of compressed air by heat exchange with returning product streams to a temperature suitable for its rectification. The rectification is performed in a so-called double rectification column comprising a higher pressure and a lower pressure rectification column, i.e. one of the two columns operates at a higher pressure than the other. Most of the incoming air is introduced into the higher pressure column and is separated into oxygen-enriched liquid air and a nitrogen vapour. The nitrogen vapour is condensed. A part of the condensate is used as liquid reflux in the higher pressure column. Oxygen-enriched liquid is withdrawn from the bottom of the higher pressure column and is used to form a feed stream to the lower pressure column. Typically, the oxygen-enriched liquid stream is sub-cooled and is introduced into an intermediate region of the lower pressure column through a throttling or pressure reduction valve. The oxygen-enriched liquid air is separated into substantially pure oxygen and nitrogen in the lower pressure column. Gaseous oxygen and nitrogen products are taken from the lower pressure column and typically form the returning streams against which the incoming air stream is heat exchanged. If desired, the gaseous oxygen product can be formed by employing air to vaporise a liquid oxygen stream withdrawn from the lower pressure column. Liquid reflux for the lower pressure column is provided by taking the remainder of the condensate from the higher pressure column, sub-cooling it and passing it into the top of the lower pressure column through a throttling or pressure reducing valve.
In the lower pressure rectification column there is created below the oxygen-enriched liquid air feed a local maximum concentration of argon. If it is desired to produce an argon product, an argon-enriched oxygen stream is withdrawn from below the level of the oxygen-enriched liquid air feed and is introduced into a further rectification column in which a crude argon product is separated from the argon-enriched oxygen, this crude argon product being taken from the top of the further column. Oxygen-enriched liquid is returned from the bottom of the further column to the lower pressure rectification column. If desired, one or both of a liquid oxygen and a liquid nitrogen product may be produced in addition to the gaseous oxygen and nitrogen product.
In order to meet the refrigeration requirements of the air separation process, a stream of incoming air or returning nitrogen is expanded in a turbine with the performance of external work. For example, a pan of the incoming air is taken out of heat exchange relationship with the returning nitrogen and oxygen streams and is expanded in a single expansion turbine to the pressure of the lower pressure column. The air so expanded is introduced into the lower pressure column. If more than about 5% of oxygen product of the lower pressure rectification column is collected in liquid state, it is typically desirable to employ a second expansion turbine which has an outlet temperature approximately equal to the inlet temperature of the first turbine. Use of such a second or `warm` turbine enables there to be maintained a relatively close match between the enthalpy-temperature profile of the air being cooled and that of the streams being warmed, thereby making for efficient heat exchange between the incoming air and the returning oxygen and nitrogen streams. The proportion of the incoming air that flows through the first turbine depends on the proportion of the oxygen and nitrogen products that are collected in liquid state. The greater this latter proportion, the greater the proportion of the incoming air that flows through the first turbine. Increasing the proportion of air that is expanded in the first turbine, and hence fed to the lower pressure rectification column, beyond a certain optimum, reduces the amount of separation that is achieved in the lower pressure rectification column, and as a result reduces the amount of argon that is recovered. On the other hand, if the air is fed by the first turbine into the higher pressure rectification column, there is a greater requirement for compression work with the result that the power consumption for a given argon recovery is similarly increased. Accordingly, In the above-described air separation process it is not generally possible to produce efficiently argon at a yield of 80% or greater if a total of about 15 mole percent or more of the oxygen and nitrogen products are collected in liquid state.
It is an aim of the present invention to provide a method and apparatus for separating air which enables argon to be produced at a yield of about 80% if not less than 15% in total of the oxygen and nitrogen products are collected in liquid state.